The present invention generally relates to an automatic lensmeter and, more particularly, to a mechanism for displaying an alignment target indicative of the position of a lens to be examined relative to the optical axis of a measurement optical system.
There have been so far proposed various sorts of lensmeters which automatically measure optical characteristics of ophthalmic lenses for use in spectacles.
In these automatic lensmeter, a mechanism for indicating a relative position of the optical center of a lens to be examined with respect to the optical axis of a measurement optical system is indispensable. To this end, generally speaking, a target called a corona target or a cross line target is indicated on a display together with a mark indicative of the measurement optical axis.
There have so far existed several systems of displaying a relative positional relationship of the optical center of a lens to be examined with respect to the optical axis of a measurement optical system, which follow.
In a first display system, prism power (prism diopter) occurred when the optical axis of the measurement optical system is shifted from the optical center of the lens to be examined is measured and the position of the target is indicated in terms of the magnitude of the measured prism power. An optical aberration has a correlative relationship with the diopter. However, it is advantageous to indicate a shift between the optical axis of the measurement optical system and the optical center of the lens to be examined in the form of prism power, so that the range guaranteed to measure accurately the optical characteristics can be set with use of a standardized reference.
In a second display system, a deviation (distance) between the optical center of a lens to be examined and the optical axis of a measurement optical system is calculated and the position of a target is indicated in terms of the calculated deviation. According to this system, the movement of the lens to be examined and the movement of the target can be made advantageously proportional to each other.
With the first display system based on the prism power, since a lens having large diopter varies greatly in prism power even in the vicinity of its optical center, it is disadvantageously difficult to attain alignment.
The first display system is also defective from the viewpoint of its manufacturing cost in the more accurate alignment cannot be obtained without increasing the resolution of the display.
The second display system of positioning and displaying the target at the position proportional to the deviation between the optical center of the lens to be examined and the optical axis of the measurement optical system, on the other hand, has such a problem that, when the standardized range is set so as to be guarantee the measurement accuracy, a lens having large diopter must be employed as a reference and thus strict positioning is required even in a lens having small diopter.
Further, accurate positioning involves the need for increasing the resolution of the display or the need for increasing its magnification. The former results in a high manufacturing cost, while the latter results in reduction of the display range.
There have been conventionally proposed various sorts of automatic lensmeters which allow marking or the lens.
The marking is usually carried out at the optical center. However, it is convenient in prism prescription to perform marking at a position where a prism to be prescribed is added. To this end, when it is desired to perform marking at a position other than the optical center, it has been conventionally common practice to move the lens in such a manner as to make the measured prism power indicated on the display visually coincide with the prescribed value, at which coincided position the axis marking has been conducted.
The above marking method, however, has been defective in that an operator must greatly resort to his experience and therefore it takes a lot of time in the marking work. Further, in the case of an astigmatic lens, the lens must be moved so as to obtain a coincidence of even its cylindrical axial angle, thus resulting in very hard marking work.